Driver control circuits are often used to actuate power switching devices, such as MOSFETs, IGBTs, etc., in switching power supplies for lighting systems and other power conversion applications, while isolating the switching control circuitry from the high voltages of the power conversion circuits. Many conventional gate drive controller circuits, however, suffer from poor performance and inability to reliably provide drive voltages sufficient to actuate the power MOSFET at high duty cycles, and certain approaches to solve this problem suffer from driver isolation transformer core saturation.
FIG. 1A shows portions of a power converter in which a buck-boost DC-DC converter circuit 300 has one or more power switches driven by isolated driver circuitry. In this system, the converter includes a buck converter stage 310 followed by a boost converter 320. The buck converter 310 receives a DC input at input terminals Vin+ and Vin−, and includes a MOSFET Q1 with a drain connected to Vin+ and a source connected to the high side input of the boost converter 320, as well as a diode D2 connected across the bock converter output lines. The boost converter 320 has a series inductor L connected to the upper input and a second MOSFET Q2 connected between the inductor L and the lower terminal, as well as a diode D3 connected from the common node of the inductor L and Q2 to an upper output Vout, with a pair of series output capacitors C2 and C3 connected between Vout and the lower terminal.
The conventional high side drive controller circuit 100a in FIG. 1A is used to drive the gate-source voltage Vgs of Q1 for operation of the buck stage 310. The controller 100a receives power (Vcc) from a power up circuit 200a and includes a transformer T1 for isolating the driver switching controls not shown) from the potentially high voltages of the buck-boost converter 300. A PWM controller 110 selectively provides a square wave to drive the transformer primary circuit including a transformer primary, a DC blocking capacitor C1, and a resistance R1 with a square wave signal. The PWM signal from the controller component 110 is coupled to the transformer primary through a resistor R1 and a series capacitor C1, and the transformer secondary provides isolated AC power to a rectifier circuit including a diode D1 and resistor Rgs to selectively provide a gate control signal (Vgs) as a voltage between gate and source terminals of the transistor Q1 in the buck converter 310. The illustrated controller 100a, however, suffers from inability to provide the necessary gate drive voltage at high PWM duty cycle levels, and thus may not be able to reliable switch the MOSFET Q1 on.
FIG. 1B shows another conventional driver circuit 100b with a DC blocking capacitor C2 added to the upper secondary circuit node to address the voltage sufficiency issues with the design of FIG. 1A for high duty cycle conditions. Although this approach is an improvement in steady state operation, if the PWM controller 110 stays on too long or shuts off, the driver circuit transformer T1 can become saturated, leading to the charge from the secondary capacitor C2 inadvertently turning on the MOSFET Q1. In particular, some PWM controllers such as the L6562 will keep the output at a high level all the time if the circuit output is lower than a setpoint voltage. If the drive circuit as shown in FIG. 1B is used to drive Q1 as shown in FIG. 1A, then after several micro seconds the gate drive transformer T1 will become saturated, Q1 will turn off and the input cannot transfer energy to the load. This, in turn, may lead to the Vcc of an L6562 controller 110 falling, with the PWM controller 110 stopping. With the PWM controller 110 off, the primary side DC blocking capacitor C1 will transfer the energy to the secondary side, resulting in Q1 being turned on, which can cause Q1 to fail. Another potential problem in the circuit 100b of FIG. 1B is if the capacitance of C1 is large and the transformer is not saturated, the DC blocking capacitor C2 of the secondary circuit may discharge slowly when controller 110 shuts off, which may lead to the FET Q1 turning on.
FIG. 1C shows a further conventional driver design 100c in which a secondary-side MOSFET Q0 has been added to the lower secondary circuit branch, with a control gate tied to the upper secondary winding. This design uses the transistor Q0 to control transformer saturation, as described in U.S. Pat. No. 6,807,071, incorporated herein by reference. However, this approach introduces an additional MOSFET component and thus increases the circuit size and cost.
As shown in FIG. 1A, the PWM controller 110 receives power (Vcc) from the power up circuit 200a. In the case of FIG. 1A, a charge-pump type power up circuit 200a with transformer T2 is used to generate Vcc, but this circuit suffers from poor output stability for load changes and/or where different AC input voltages are received from the AC input source 210.
FIG. 1D is a partial schematic diagram illustrating a full wave rectifier power up circuit 200b that can be used, but this design also fails to provide steady supply voltages Vcc to the controller 110 and other integrated circuits as input supply levels and/or output loading conditions change.
Thus, there remains a need for improved gate driver circuits and power up circuits to provide switching control circuit isolation from driven switching devices while mitigating isolation transformer core saturation.